1. Field of the Invention
The present invention relates to an image display apparatus, and more particularly to a flat image display apparatus that uses electron-emitting elements.
2. Description of the Related Art
In recent years, flat displays have been developed as next-generation displays, in which a number of electron-emitting elements are arranged and opposed to the phosphor screen. Various types of electron-emitting elements are available. Basically, they perform electric-field emission. Any display using electron-emitting elements is generally called a field-emission display (hereinafter referred to as an FED). Of the various FEDs available, a display that uses surface-conduction electron-emitting elements is called a surface-conduction electron emission display (hereinafter referred to as an SED). Nonetheless, an SED will be referred to as an FED in the present application.
An FED has a front substrate and a rear substrate, which are opposed to each other and spaced apart by a narrow gap of about 1 to 2 mm. These substrates are fused at their peripheral edges, with a rectangular frame-shaped side wall interposed between them. The substrates therefore form a vacuum envelope. The interior of the vacuum envelope is maintained at a high vacuum of about 10−4 Pa. A plurality of spacers are provided between the substrates, supporting the substrates against the atmospheric pressure applied to them.
On the inner surface of the front substrate, a phosphor screen including red, blue and green phosphor layers is formed. On the inner surface of the rear substrate, a number of electron-emitting elements are provided. These elements emit electrons, which excite the phosphors and make them emit light. On the rear substrate, a number of scanning lines and a number of signal lines are provided, in the form of a matrix. These lines are connected to the electron-emitting elements. An anode voltage is applied to the phosphor screen, accelerating the electron beams emitted from the electron-emitting elements. The electrons thus accelerated impinge on the phosphor screen. The screen therefore emits light, whereby the FED displays an image.
In the FED described above, phosphor of the same type as the one used in the ordinary cathode ray tube is used in order to provide practical display characteristics. Further, the phosphor screen must have an aluminum film called a metal back, which covers the phosphor. In this case, the anode voltage applied to the phosphor screen is preferably at least several kilovolts (kV), or 10 kV or more if possible.
However, the gap between the front substrate and the rear substrate cannot be made so large, in view of the desired resolution and the characteristic of the spacers. The gap is therefore set to about 1 to 2 mm. Hence, an intense electric field is inevitably applied in the gap between the front substrate and the rear substrate in the FED. Consequently, discharge, if any, between these substrates becomes a problem.
If no measures are taken against possible damage due to discharge, the discharge will degrade or destroy the electron-emitting elements, the phosphor screen, the driver IC and the drive circuit. Possible damage to these components will be generally called discharge damage. In any condition where discharge damage may occur, discharge should be avoided, by all means, for a long time in order to make the FED a practical apparatus. This is, however, very difficult to achieve in practice.
It is therefore important to reduce the discharge current to such a level as would cause no discharge damage or would cause but negligibly small discharge damage, even if a discharge takes place. Known as a technique of reducing the discharge current is dividing the metal back into segments. Depending on its configuration, the FED may have a getter layer on the metal back in order to maintain a desired degree of vacuum. In this case, the getter needs to be divided into segments, too. For convenience, terms “metal back dividing” and “divided metal back” will be used hereinafter.
Metal back dividing can be classified mainly into two types. One is one-dimensional dividing, i.e., dividing the metal back, in one direction, into strip-shaped segments. The other is two-dimensional dividing, i.e., dividing the metal back, in two directions, into island-shaped segments. The two-dimensional dividing can reduce the discharge current more than the one-dimensional dividing. Jpn. Pat. Appln. KOKAI Publication No. 10-326583 (hereinafter referred to as Patent Document 1), for example, discloses the basic concept of one-dimensional dividing. Jpn. Pat. Appln. KOKAI Publication No. 2001-243893 (hereinafter referred to as Patent Document 2) and Jpn. Pat. Appln. KOKAI Publication No. 2004-158232 (hereinafter referred to as Patent Document 3) disclose two-dimensional dividing.
If the metal back is divided into segments, it is necessary to provide a path for the beam current, to reduce the luminance decrease to a tolerable level and to prevent discharge due to the potential difference at the gap. In connection with this point, Patent Document 1 and Patent Document 3 disclose a configuration in which a resistance layer is provided between the metal-back segments. Patent Document 2 discloses a configuration in which the metal-back segments are connected to power lines by resistance layers. The technique of providing resistance layers between the metal-back segments is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-251797, too.
To maintain a sufficient degree of vacuum in the envelope of the FED of the configuration described above, a getter film may be provided on the metal back in some cases. In the two-dimensional dividing, too, a getter film may be divided into segments by using projections and depressions made on and in the surface, as is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-068237 and Jpn. Pat. Appln. KOKAI Publication No. 2004-335346.
In any conventional configuration in which the metal back is divided into segments, the following three requirements must be accomplished. (1) The discharge current should be equal to or smaller than the tolerance current. (2) The gaps between the metal-back segments should serve as resistors, and the anode current should decrease as the beam current flows through these resistors. (3) No discharge should occur, resulting from the voltage generate in the gaps between the metal-back segments, at the time of discharge.
In the configuration described in, for example, Patent Document 2, wherein the metal-back segments are connected to power lines, respectively, the discharge current may indeed be decreased, but to a limited value. The problems with the prior art, which should be solved, will be explained below, on the assumption that resistor layers are provided between the metal-back segments as is disclosed in Patent Document 1 and Patent Document 3.
The electrical parameter important to the two-dimensional division is resistance Rx between the metal-back segments arranged in X direction and resistance Ry between the metal-back segments arranged in X direction. In a typical rectangular screen that is longer in the horizontal direction than in the vertical direction, the X and Y directions are the major-axis direction and the minor-axis direction, respectively. Nevertheless, the general definition of the X and Y directions will be described later.
In order to achieve the requirement (1) described above, it is advantageous to increase Rx and Ry. To achieve the requirements (2) and (3), it is useful to decrease Rx and Ry. Thus, the requirement (1), on the one hand, and the requirements (2) and (3), on the other, are in a trade-off relation. Inevitably, the discharge current cannot be reduced as much as desired.
Therefore, there has been a demand for a technique that can reduce the discharge current as much as desired.